1. Field of the Invention
The present invention relates to a field emission device, and more particularly to a field emission device using carbon nanotubes. The application relates to a contemporaneously filed application having the same applicant and the same assignee with the instant invention, and titled “METHOD FOR PROCESSING ONE-DIMENSIONAL NANO-MATERIALS”.
2. Description of Prior Art
Carbon nanotubes were first discovered and reported in an article by Sumio Iijima entitled “Helical Microtubules of Graphitic Carbon” (Nature, Vol. 354, Nov. 7, 1991, pp. 56–58). Carbon nanotubes have superior electron emission capability at low emission voltages, generally less than 100 volts. Furthermore, carbon nanotubes can carry high electric currents reliably. Due to these properties, carbon nanotubes are considered to be an ideal field emission material for applications in a variety of display devices including flat panel displays, such as field emission displays.
Existing carbon nanotube synthesis techniques include arc discharge, laser vaporization, and chemical vapor deposition (CVD). Carbon nanotubes formed by any of these methods alone cannot satisfactorily be used as field emission material for the following reasons. Carbon nanotubes formed using the arc discharge and laser vaporization methods have non-uniform heights and orientations, and are prone to be tangled together. If the carbon nanotubes are directly used as field emission material, they tend to reduce field concentration and field efficiency. Carbon nanotubes formed using the chemical vapor deposition method have uniform height and are well aligned. However, the high density of carbon nanotubes formed is inclined to induce shielding between adjacent carbon nanotubes. In addition, cavities at the tips of carbon nanotubes are encapsulated by catalytic metal particles, thereby reducing field concentration and efficiency.
For carbon nanotubes to be successfully applied in electronic devices, their electron emission properties must be optimized. This can be done by processing the carbon nanotubes. Many techniques have been devised to improve the field concentration and efficiency of carbon nanotubes. These techniques include opening the tips of the carbon nanotubes, purifying the carbon nanotubes, and re-orienting the carbon nanotubes.
A method for opening tips of carbon nanotubes and purifying the carbon nanotubes is disclosed in China patent application CN1292354A. FIG. 7 is a schematic sectional illustration of the method, in which a laser beam is used to open the tips and purify the carbon nanotubes. The method includes the following steps: (1) aligning crude carbon nanotubes 112 perpendicularly on a substrate 110; (2) irradiating the crude carbon nanotubes 112 with a laser beam 140 generated by a laser generator 144, the irradiation occurring at a predetermined height in a direction parallel to a major surface of the substrate 110 to cut off the tips from the crude carbon nanotubes 112; and (3) removing the tips from the crude carbon nanotubes 112. However, the laser generator 144 must be accurately adjusted to ensure that the crude carbon nanotubes 112 are irradiated by the laser beam 140 at the correct height. This makes the method unduly time-consuming. Furthermore, after processing, the carbon nanotubes being used as a field emission device are still densely configured. This induces shielding between adjacent carbon nanotubes, reduces field concentration and efficiency, and increases a threshold voltage of field emission.